20W-50 in a GM 8 cylinder with 160 K miles is a good summer oil. It might cost you a little in MPG's, but with 160 K miles any XW-30 oil is probably too thin. Often when you get that kind of miles on a old GM 8 cylinder you have to use a 40 or a 50 grade as the top number.
I have Castrol GTX 20W-50 in my 1985 Olds Delts 88 with over 160 K miles on it. I had to go to something thicker than a XW-30 because it burned and leaked 30 too fast, and it would sometimes stall on a very hot day. With the 20W-50 I do not have to add oil as often, and I do not have to smell the leaking oil burning off, and it does not stall on hot days.
With that kink of miles the clearance between parts is more than it was when new, and a thicker oil works better.
I am thinking of going with something like the newer 5W-50 oil for winter. Right now with a 20W-50 in it, 15 Fahrenheit is about the coldest that I would have the temperature be and still try to start it.